Estimated at about 1,000 miles (1,600 kilometers) wide, the giant asteroid is believed to have struck Mars's northern hemisphere billions of years ago.

The cataclysm is thought to have given the planet its topographical split personality—smooth in the north but bumpy down south, generally speaking.

The impact site became the smooth, low-lying Borealis Basin, about 6,000 miles (10,000 kilometers) across. The southern part of the planet became highlands—in places several miles higher than the basin.

The border of the two regions is sharply defined, except for the Arabia Terra zone. This odd middle ground is neither highlands nor basin.

Until recently, the reason for the region had been unknown.

Impact Relic

Arabia Terra a relic of the giant asteroid impact, geophysicist Jeff Andrews-Hanna, of the Colorado School of Mines, suggested in December at an American Geophysical Union meeting in San Francisco.

This unusual midland was created when a U.S.-size portion of the highlands broke free and slid 180 miles (300 kilometers) northward, down into the southern rim of the Borealis Basin, Andrews-Hanna said.

In other words, three of Mars's largest geographic features—the Borealis Basin, the highlands, and Arabia Terra—were formed "virtually instantaneously, in a single catastrophic collision," the geophysicist said via email.